Methane yields and methanogenic community changes during co-fermentation of cattle slurry with empty fruit bunches of oil palm

The biomethane potential and structural changes of the methanogenic community in a solid-state anaerobic digestion process co-digesting cattle slurry and empty fruit bunches were investigated under mesophilic (37 °C) and thermophilic (55 °C) conditions. Phylogenetic microarrays revealed the presence of two hydrogenotrophic genera (Methanoculleus and Methanobrevibacter) and one acetoclastic genus (Methanosarcina). Methanosarcina numbers were found to increase in both mesophilic and thermophilic treatments of empty fruit bunches. Methanobrevibacter, which dominated in the cattle slurry, remained constant during anaerobic digestion (AD) at 37 °C and decreased in numbers during digestion at 55 °C. Numbers of Methanoculleus remained constant at 37 °C and increased during the thermophilic digestion. Physicochemical data revealed non-critical concentrations for important monitoring parameters such as total ammonia nitrogen, free ammonia nitrogen and volatile fatty acids in all treatments after AD. The biomethane potential of empty fruit bunches was higher under thermophilic conditions than under mesophilic conditions.